Combined effects on residual strength of a high performance concrete exposed to fire

Concrete structures exposed to fire suffer from damage, but can remain a certain degree of residual strength. International research has shown that the compressive strength of concrete decreases not only with temperature, but also by the way of cooling and the storage conditions after fire. Fast cooling introduces a thermal shock which, based on experiments by the authors, could result in a 30% additional strength loss with respect to the loss during heating. When storing the concrete after the fire in air or under water, additional strength losses of about 20-30 % are found within 14 days after the fire. In this paper it is investigated for a high performance concrete what the combined effect is of heating, cooling and storage. One of the conclusions – but with respect to the specific test conditions (e.g. slow heating, 550°C max, pre-dried samples) – is that superposing both expected strength losses of about 30% in case a fast cooling is followed by a period of post-cooling storage results in too conservative strength estimations. It is deemed that the cracks resulting from fast cooling, will act as expansion chambers for the newly produced portlandite, thus strongly reducing additional stresses, which results in expected lesser damage.

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